1. Field of the Invention
The present invention relates to heaters for heat fixing devices that fix toner to sheets by heating.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2005-71843 discloses a plate heater including a substrate, a heating element disposed on the substrate, and an overcoat glass layer (protective film) disposed over the heating element.
The substrate disclosed in the above literature is a glass substrate formed from a glass material containing SiO2, Al2O3, and Li2O. The overcoat glass layer is formed from a glass material containing a low-melting-point glass and a low-expansion filler. A Teflon® coating is formed over the overcoat glass layer.
Unfortunately, the heater disclosed in the above literature has a problem in that alkali metals contained in the glass materials may migrate between electrode patterns connected to the heating element, depending on the arrangement of the electrode patterns.
Another problem with the heater disclosed in the above literature is that the heater may warp because it does not take into account the difference in thermal expansion coefficient between the substrate and the protective film (overcoat glass layer).
A further problem with the heater disclosed in the above literature is that the low-expansion filler contained in the overcoat glass layer decreases the surface smoothness (i.e., increases the surface roughness) of the overcoat glass layer. If a heating belt comes into contact with such an overcoat glass layer, the heating belt may be damaged by large irregularities in the surface of the overcoat glass layer.